While the principle of utilizing air pressure to support and guide a load as it moves along a track is not new, improvements have been realized in load-carrying efficiency through the implementation of a system using rail sections having a concave support surface through which small diameter nozzles extend, the nozzles being angled with respect to both longitudinal and transverse planes. Air exiting the nozzles under pressure, provided in a plenum below the rail, creates a thin film between the rail surface and load support member positioned in the rail. The thin film of pressurized air creates a hydrostatic bearing on which the load support member can ride so that the member and the load supported thereby can move easily as it is guided along the rail. Usually a pair of rails will be positioned side by side to define a track system, there being a load supporting member located in each rail of the track system so as to support a carrier or deck which spans the gauge distance between the rails and is adapted to carry the desired load. A palletized load can be placed on the deck for movement along the track system.
Load carrying systems of the aforementioned type are sold commercially under the trade-mark "SAILRAIL" by Sailrail Enterprises, a division of E. B. Eddy Forest Products Ltd. Various patents relate to the system per se, the design of the rails, and the design of the runners, including U.S. Pat. Nos. 3,873,163; 4,489,825; 4,550,823; 4,579,320; 4,616,960; 4,687,079 and 4,887,706.
The two major components of the SAILRAIL system are the rail, or track, and the load support member, or runner. The runner must meet several criteria in order to be effective. It must be compliant so as to aid in the proper distribution of the air film and thus work on rails that have profiles that are slightly variable or imperfect due to wear, dirt or manufacturing tolerances; it must be abrasive resistant and tough so that replacement is not required very often; it must be reasonably light since runners with decks attached may be lifted by hand to or from the rails; it must be easily attached to or removed from a deck; it must work efficiently in the rail system for which it is designed; and it must be economical to manufacture and maintain.
Heretofore runners for SAILRAIL systems have been manufactured in lengths of about four feet using a "log", being a roll of cellulosic paper product wound tightly on a collapsible cardboard core. The paper product was quite strong in tensile and had some stretch capability as a result of limited crepe. The web product resembled commercial serviette or napkin stock and is produced on the same production machinery that makes paper bathroom and towelling rolls. For the SAILRAIL product, however, the wound circular log was first of all compressed while positioned on a curved plate of rail dimensions under a load of about 8000 pounds to create a generally oval shape in transverse cross section in which the upper surface is flatter than the curved lower surface. The deformed log was then wrapped in a cover which was smooth, tough, compliant, abrasion resistant and moisture-proof. The cover could be, for example, a high density polyethylene with additives such as T.sub.i O.sub.2 to improve wear properties. The cover was closed over the log to secure the log, to provide the reactive bearing surface, and the resulting runner was ready for attachment to and subsequent use with a deck. The lower curvature was very close to that of the rail surface; the upper surface was generally flat for attachment to the underside of the deck; and the combination of the collapsed core, the squashed cellulosic material and the cover provided a runner that was compliant, capable of limited deformation under the application of a load and capable of at least partial recovery after load removal.
Runners manufactured as above are very effective and do exhibit vibration cushioning properties, but they are expensive to manufacture and because of their bulk are somewhat limited in their applicability to situations in which less exposure would be desirable. There is thus a need for a more versatile and less expensive runner to make the SAILRAIL system more attractive to potential users of the system.
An improvement on the above-described runners is taught in U.S. Pat. No. 4,838,169 which describes a runner having an elongated core, the core having a generally flat upper surface and a generally transversely convex lower surface. A pad of flexible, compliant material covers at least the lower surface and an outer cover generally encloses the pad and core. The pad and core are formed of material that is capable of deformation under load and at least partial recovery when the load is removed. The runner of U.S. Pat. No. 4,838,169 uses less pad material than previous runners but, even with transverse slots cut in the upper surface thereof it does not exhibit sufficient flexibility for use with all rails under all conditions.